Graphene Oxide. Origin of Acidity, Its Instability in Water, and a New Dynamic Structural Model

Abstract

The existing structural models of graphene oxide (GO) contradict each other and cannot adequately explain the acidity of its aqueous solutions. Inadequate understanding of chemical structure can lead to a misinterpretation of observed experimental phenomena. Understanding the chemistry and structure of GO should enable new functionalization protocols while explaining GO's limitations due to its water instability. Here we propose an unconventional view of GO chemistry and develop the corresponding "dynamic structural model" (DSM). In contrast to previously proposed models, the DSM considers GO as a system, constantly changing its chemical structure due to interaction with water. Using potentiometric titration, (13)C NMR, FTIR, UV-vis, X-ray photoelectron microscopy, thermogravimetric analysis, and scanning electron microscopy we show that GO does not contain any significant quantity of preexisting acidic functional groups, but gradually generates them through interaction with water. The reaction with water results in C-C bond cleavage, formation of vinylogous carboxylic acids, and the generation of protons. An electrical double layer formed at the GO interface in aqueous solutions plays an important role in the observed GO chemistry. Prolonged exposure to water gradually degrades GO flakes converting them into humic acid-like structures. The proposed DSM provides an explanation for the acidity of GO aqueous solutions and accounts for most of the known spectroscopic and experimental data.

Towards the understanding of the graphene oxide structure: How to control the formation of humic- and fulvic-like oxidized debris

“…More recently, the degradation of GO in water has been reported to be associated with the formation of humic-like entities [40]. In soil science, humic and fulvic acids, together with humin, are formed by the degradation of biomolecules.…”

An investigation into the colloidal stability of graphene oxide nano-layers in alite paste

“…(mmol/g) which is 5-8 mmol/g according to [34]. This leads to a theo- lower than what GO could potentially provide in the paste.…”

Estudo Através De RMN De 13c No Estado Sólido Sobre a Síntese De Oxido De Grafite Utilizando Diferentes Precursores Grafíticos

“…15 Com base nos trabalhos de Rourke e Dimiev, novos estudos propuseram que 1/3 da composição de OG é de folhas de grafeno oxidadas contendo grupos hidroxila e epóxidos no interior do plano basal e carboxilas e lactois nas extremidades do plano. Os 2/3 restantes são referentes a ODs envoltos por moléculas de água e aderidos às folhas de óxido de grafeno, como ilustrado na Figura 2.…”